1. Field of the Invention
The invention relates to a method for chemically grinding a semiconductor wafer on both sides. In particular, the invention is directed to a novel processing step for silicon wafers of the next technology generations, primarily having a diameter of 450 mm.
2. Background Art
At present, polished or epitaxially coated silicon wafers having a diameter of 300 mm are used for the most demanding applications in the electronics industry. The silicon wafers having a diameter of 200 mm are gradually being superseded by the 300 mm wafers.
A major reason why the electronics industry demands larger substrates for producing the components in this industry, be they microprocessors or memory chips, resides in the enormous economic advantage that this affords. In the semiconductor industry it has long been customary to focus attention on the available substrate area, or in other words to ask the question of how high is the number of components, that is to say logic chips or memory chips, which can be accommodated on an individual substrate. This is associated with the fact that a large number of processing steps by the component manufacturers are directed to the entire substrate, but also in addition, to individual steps for patterning the substrates, that is to say the production of component structures that subsequently lead to the individual chips. Thus, both groups of processing steps are closely related to the substrate size with regard to the production costs. The substrate size thus considerably influences the production costs per component and is thus of immense economic importance.
However, an increase in the substrate diameter is accompanied by major, in part also totally new, hitherto unknown technical problems.
Ultimately, all the processing steps, whether they be purely mechanical (sawing, grinding, lapping), chemical (etching, cleaning) or else chemical-mechanical in nature (polishing), and also the thermal processes (epitaxial coating, annealing) require thorough reworking, in particular also with regard to the machines and installations used therefor (equipment).
Chemomechanical processing comprises polishing methods in which a material removal is obtained by means of relative movement of semiconductor wafer and polishing pad with the action of force and with the supply of a polishing slurry (for example alkaline silica sol). The prior art describes batch double-side polishing (DSP) and batch and individual wafer single-side polishing (mounting of the semiconductor wafers by means of vacuum, adhesive bonding or adhesion during the polishing processing on one side on a support).
Mechanical processing steps in accordance with the prior art are lapping (simultaneous double-side lapping of a plurality of semiconductor wafers in the “batch”), single-side grinding of individual semiconductor wafers with single-side clamping of the workpieces (usually carried out as sequential single-side grinding of both sides of the wafer, “single-side grinding”, SSG; “sequential SSG”) or simultaneous double-side grinding of individual semiconductor wafers between two grinding disks (simultaneous “double-disk grinding”, DDG).
Methods and apparatuses for the single-side surface grinding of a semiconductor wafer are known for example from U.S. Pat. No. 3,905,162 and also U.S. Pat. No. 5,400,548 or EP-0955126. In this case, a semiconductor wafer is fixedly held on a wafer holder by one of its surfaces, while its opposite surface is processed by means of a grinding disk by wafer holder and grinding disk rotating and being pressed against one another. In this case, the semiconductor wafer is fixed on the wafer holder in such a way that its center substantially corresponds to the rotation center of the wafer holder. Moreover, the grinding disk is positioned in such a way that the rotation center of the semiconductor wafer presses into a working region or the edge region of the grinding disk, the edge region being formed by teeth. The entire surface of the semiconductor wafer can thereby be ground without any movement in the grinding plane.
DDG machines according to the prior art, as are described for example in JP2000-280155A and JP2002-307303A, have two grinding wheels which lie opposite one another and the rotation axes of which are arranged colinearly. During the grinding operation, a workpiece in wafer form which is positioned between the grinding wheels is processed on both sides simultaneously by the two grinding wheels rotating about their axis, while it is held in position by a ring-shaped holding and rotation device and at the same time rotated about its own axis. During the grinding operation, the two grinding wheels are advanced in an axial direction until the desired final thickness of the workpiece has been reached.
The holding and rotation device may, for example, comprise friction wheels which engage on the edge of the workpiece. However, it may also be a device which surrounds the workpiece in ring-shaped fashion and engages in a score, groove or notch which is possibly present at the periphery of the workpiece. A device of this type is generally referred to as a “notch finger”. In order to process the entire area of the workpiece, the workpiece is guided relative to the grinding wheels in such a way that the abrasive grinding segments of the grinding wheels describe a circular path which runs constantly over the center of the workpiece.
In this case, the workpiece is not generally in a fixed position, but rather is held axially in position by two apparatuses for hydrostatic bearing, referred to hereinafter as “hydropads”, which are fitted on both sides of the workpiece. Apparatuses of this type are described in JP2000-280155A. In accordance with the prior art, those surfaces of the two hydropads which face the workpiece are configured in planar fashion and oriented parallel to one another. Each hydropad comprises a plurality of hydrostatic bearings, between which grooves for discharging the medium used for the hydrostatic bearing (referred to hereinafter as the “hydro-bearing medium”) and the grinding coolant are arranged.
For producing particularly planar semiconductor wafers, great importance is ascribed to those processing steps in which the semiconductor wafers are processed largely in a constrained-force-free manner in “free-floating” fashion without force-locking or positively locking clamping (“free-floating processing”, FFP). Undulations such as are produced for example by thermal drift or alternating load in MWS are eliminated by FFP particularly rapidly and with little loss of material. FFP processes known in the prior art include lapping, DDG and DSP, inter alia.
DE 103 44 602 A1 discloses a further mechanical FFP processing method, in which a plurality of semiconductor wafers lie in a respective cutout in one of a plurality of carriers that are caused to rotate by means of a ring-shaped outer and a ring-shaped inner drive ring, and are thereby held by a specific geometrical path and processed in material-removing fashion between two rotating working disks coated with bonded abrasive. This method is also called “planetary pad grinding” or simply PPG. The abrasive is composed of a film or “pad” stuck to the working disks of the apparatus used, as disclosed in U.S. Pat. No. 6,007,407, for example.
Hard materials are used as the abrasive, e.g. diamond, silicon carbide (SiC), cubic boron nitride (CBN), silicon nitride (Si3N4), cerium dioxide (CeO2), zirconium dioxide (ZrO2), corundum/aluminum oxide/sapphire (Al2O3) and many other ceramics having grain sizes up to a few tens of micrometers. For the processing of silicon, in particular diamond is preferred, and furthermore also Al2O3, SiC and ZrO2. The diamond is incorporated, as individual grains, or bonded by means of a ceramic, metallic or synthetic resin primary bond to form conglomerates, into the ceramic, metal or synthetic resin matrix of the abrasive bodies.
DE 103 44 602 A1 furthermore discloses a method in which either a multiplicity of abrasive bodies containing bonded abrasive are stuck to the working disks or in which the abrasive is bonded in a layer or a “pad” and pads of this type are stuck to the working disk. There are furthermore fixings of the working layer by means of vacuum, screwing, covering or by means of hook and loop fastening, in electrostatic or magnetic fashion (see e.g. U.S. Pat. No. 6,019,672 A).
Sometimes the working layers are embodied as pads or laminated sheets (U.S. Pat. No. 6,096,107 A, U.S. Pat. No. 6,599,177 B2). Sheets having structured surfaces are also known, comprising elevated regions that come into contact with the workpiece and recessed regions via which cooling lubricant can be supplied and abrasive slurry and spent grain can be discharged. An abrasive tool (abrasive pad) structured in this way is disclosed by U.S. Pat. No. 6,007,407 A, for example. Here the abrasive pad is self-adhesive on the rear side, which permits a simple change of the abrasive tool on the working disk.
Suitable apparatuses for carrying out the processing methods of lapping, DSP and PPG essentially comprise a ring-shaped upper and lower working disks and a rolling apparatus comprising toothed rings arranged on the inner edge and on the outer edge of the ring-shaped working disks. Upper and lower working disks and inner and outer toothed rings are arranged concentrically and have colinear drive axes. The workpieces are introduced into thin guide cages which are toothed on the outside, so-called “carriers”, which are moved between the two working disks during processing by means of the rolling apparatus. In the case of PPG, the working disks comprise, as mentioned above, a working layer with fixedly bonded abrasive.
In the case of lapping, use is made of working disks, so-called lapping plates, composed of cast material, generally a steel casting, e.g. ductile gray cast iron. These contain in addition to iron and carbon a multiplicity of nonferrous metals in different concentrations.
In the case of DSP, the working disks are covered with a polishing pad, wherein the polishing pad is composed for example of a thermoplastic or heat-curable polymer. A foamed plate or a felt or fiber substrate which is impregnated with a polymer is also suitable. In the case of lapping and DSP, lapping and polishing agents, respectively, are additionally supplied, but not in the case of PPG. For lapping, oils, alcohols and glycols are known as carrier liquids for the lapping agent (abrasive substance slurry, abrasive substances), also called a “slurry”. For DSP, aqueous polishing agents to which silica sol is applied are known, which are preferably alkaline and, if appropriate, contain further additives such as chemical buffer systems, surfactants, complexing agents, alcohols and silanols.
The production of a semiconductor wafer comprises slicing the semiconductor wafer from a crystal followed by a plurality of subsequent material-removing processing steps. These processing steps are necessary in order to obtain the smoothest possible surfaces and parallel sides of the semiconductor wafer, and also to provide the semiconductor wafer with a rounded edge. Appropriate material-removing processing steps usually include edge rounding, lapping or double-side grinding, etching, and polishing of the semiconductor wafer. Processing steps such as double-side grinding and primarily lapping add damage to the wafer surface, this damage necessitating a high amount of material removal in subsequent steps (etching, polishing).
The crystal damage can be reduced by fine grinding of the semiconductor wafer, that is to say by surface grinding using a grinding disk having a fine grain size. Less material removal is then necessary during subsequent etching. Ideally, the intention is to completely dispense with the etching step. Such a reduced etching step is intended to have the effect that impairment of the flatness of the semiconductor wafer that is usually associated with etching is reduced, and less material removal is in turn necessary in the subsequent polishing step. A fine grinding method of this type is described in DE 102 005 012 446 A1.
However, fine grinding also disadvantageously influences the geometry and, in particular, also the nanotopology, which is becoming more and more problematic on account of the constantly increasing requirements made of these two parameters in the course of further miniaturization (Roadmap, Design Rules). The nanotopology is usually expressed as height fluctuation PV (“peak to valley”), relative to square measurement windows having an area of 2 mm×2 mm. Moreover, a conventional fine grinding process by which the wafer situated on a rotating wafer holder and a rotating disk are delivered to one another (advance) cannot readily be employed in view of the forces acting on 450 mm wafers according to the current situation.